The effect of annealing temperature on the electrochromic properties of nanostructured NiO films

ARTICLE IN PRESS Solar Energy Materials & Solar Cells 93 (2009) 1195–1201

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The effect of annealing temperature on the electrochromic properties of nanostructured NiO films K.K. Purushothaman, G. Muralidharan  Department of Physics, Gandhigram Rural University, Gandhigram 624302, Tamilnadu, India

a r t i c l e in fo

abstract

Article history: Received 29 July 2008 Accepted 19 December 2008 Available online 31 January 2009

The nickel oxide (NiO) films have been prepared by sol–gel dip coating route. They were treated at different temperatures ranging from 250 to 450 1C at an optimum number of layers (8 layers) on a conducting substrate (FTO) and glass plate. The X-ray diffraction (XRD) spectrum reveals that the crystallanity increases along the planes (111), (2 0 0) and (2 2 0) and the crystallite size increases from 7 to 26 nm as the temperature increased from 250 to 450 1C. The FTIR spectrum confirms the formation of Ni–O bond. The SEM images indicate the formation of nanorods in the temperature range 350–450 1C. The electrochromic properties have been studied using cyclic voltammetric (CV) and chronoamperometric (CA) techniques. The Ni(II)/Ni(III) transformation is the possible cause for the reversible colour change from transparent to dark brown. The film prepared at 300 1C with a thickness of 306 nm exhibits maximum anodic/cathodic diffusion coefficient of 11 1012 cm2/s/6.44  1012 cm2/s and the same film exhibits the maximum colour change of 68% with a photopic contrast ratio of 5.17. The chronoamperogram reveals that the colouration/decolouration response time decreases with the increase in preparation temperature. The films treated at 300 1C exhibit the optimum electrochromic behaviour. & 2008 Elsevier B.V. All rights reserved.

Keywords: NiO Electrochromics Nanostructures Sol–gel

1. Introduction Most of the transition metal oxides can be electrochemically switched using redox states that have an intense electronic absorption. The electrochromism in transition metal oxides is defined as a reversible change of the optical properties under an applied electric field [1–3]. A number of different organic and inorganic electrochromic (EC) materials have been used to improve the dynamic and static EC properties like enhancing EC contrast and shortening optical switching time. Electrochromism involve both electron conduction and ion diffusion. Electrochromic materials attract larger interest due to their varied applications like architectural glazing, automobile and building sun roofs, displays for light regulation and energy savings etc. [4–7]. Electrochromism in nickel oxide (NiO) film is well known but still there is no single accepted model for the mechanism that controls the colouring/bleaching process [8]. The generally accepted models for transition from coloured to a bleached state is reversible transformation between NiOOH and Ni(OH)2 phases, or in general to the redox process from Ni3+ to Ni2+ and vice-versa [9]. The electrochromic properties are mainly depen-

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E-mail addresses: [email protected] (K.K. Purushothaman), [email protected] (G. Muralidharan). 0927-0248/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2008.12.029

dent on the structure of the films. In recent years, nanostructured EC layer has been developed to improve the EC properties by replacing traditional EC layer [10–12]. The porous NiO films showed the change in transmission of 82% at 550 nm and the film start to degrade after 60th cycle was reported by Xia et al. [13]. Xue et al. [14] reported the EC properties of Cu-, and Fe-doped hexagonal tungsten bronze nanorods with the colour contrast of 38.7% and 46.6%, respectively. Films with nanocrystallites were reported to exhibit better electrochromism with the decrease in particle size [15]. When the particle size is small, the surface area to the volume ratio is large and the same probably lead to an increase in the EC efficiency. However, there are not many reports on the EC properties of NiO films with nanorods structure. The NiO films can be prepared using a number of techniques such as spraying, sputtering, vacuum deposition, sol–gel process, etc. [9,15–18]. The sol–gel process is a low-cost method with several advantages like ease of adding dopants, to control the thickness of the film, to obtain homogeneity on a molecular level and amenability to deposit large-area coatings. In the present work nickel oxide films were prepared on glass substrates and FTOcoated glass substrates via sol–gel route using dip coating technique. The dip coated films were annealed at different temperatures. Studies on the effect of annealing temperature on the structural and the electrochromic properties of multilayer thin films have been carried out and they are reported and discussed. To the best of our knowledge, this is the first report on the

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electrochromic behaviour of NiO nanorods via sol–gel dip coating method.

2. Experimental details Nickel oxide films were prepared by the sol–gel dip coating method. 3.7329 g of Ni(Ac)2  4H2O was dissolved in 100 ml of 2-methoxyethanol and 2 drops of conc. HCl was added to the solution. The solution was stirred at 60 1C for an hour and then aged for 24 h at room temperature. The NiO films were coated on an FTO substrate (6O/&) at a withdrawal speed of 15 cm/min. After each coating, the film (each layer) was dried in air for 5 min and thermally treated at different temperature in the range 250–450 1C for 5 min. Totally 8 layers have been coated. Finally, the film was processed for 1 h at 250–450 1C [for example, the film prepared at 250 1C would mean that each layer of the film was treated at 250 1C for 5 min and that the final multilayer film was annealed at 250 1C for 1 h]. The X-ray diffraction (XRD) of the films was recorded using CuKa radiation (1.54 A˚). The transmission spectrum of the films has been recorded in the range 190–1100 nm using a Perkin-Elmer Lambda-35 UV–vis spectrometer. The FTIR spectra have been recorded using JASCO 460PLUS. The thermogravimetric (TG) and differential thermal analysis (DTA) measurements were made using TA Instruments Model SDT Q600 thermal analyser instrument at a scan rate of 20 1C/min using a ceramic crucible. The films were subjected to electrochemical ion (H+, OH) insertion/extraction in a three-electrode cell with Pt as the counter electrode, Ag/AgCl as the reference electrode and 0.1 M KOH as the electrolyte at a scan rate of 25 mV/s. Texas CHI 6438 electrochemical analyser was used to record the cyclic voltammetry and chronoamperometric curves.

crystallization, originating from nickel acetate tetrahydrate. In the second step from 198 to 278 1C, a small weight loss of approximately 2.3% occurs, suggesting the formation of an intermediate basic acetate with a formula (1x)Ni ((CH3COO)2  xNi(OH)2) [19]. Thermal decomposition of the dehydrated intermediate takes place in the temperature range 287–366 1C. Within this step, combustion of acetate groups takes place. At the same time NiO nanoparticles are formed, as confirmed by XRD analysis. During this process, the weight loss is 45.24% is observed on a TG curve, followed by an exothermic peak on a DTA curve. The probable mechanism for the conversion of nickel acetate tetra hydrate to nickel oxide is given by NiðCH3 COOÞ2  4H2 O ! ð1  xÞNi ðCH3 COOÞ2  xNiðOHÞ2 ! yNiO þ ð1  yÞNi ! NiO where x and y depends on the heating rate and the temperature. The presence of OH and acetate groups in the FTIR of the films up to 300 1C is an evidence of this conversion process. With increase of temperature the bands due to OH and acetate groups

(200) (111)

Intensity(a.u)

1196

(220)

e d c b a

3. Results and discussions

10 3.1. Thermal properties Fig. 1 shows the thermogravimetric and differential thermal analysis curves of a dried xerogel powder under an air atmosphere. The initial weight of the xerogel was 2.478 g yielding 0.8911 g of residue after heat treatment up to 1200 1C. Dehydration, decomposition of acetate groups and the formation of nickel oxide have been observed at different temperatures. On the TG curve a weight loss of 14% up to 200 1C has been observed, accompanied by an endothermic peak positioned at 185 1C on a DTA curve. This step corresponds most likely to a loss of water of

Fig. 1. TG-DTA curve of a dried xerogel.

20

30

40 2θ/deg

50

60

70

Fig. 2. X-ray diffraction spectra of the films prepared at (a) 250 1C, (b) 300 1C, (c) 350 1C, (d) 400 1C and (e) 450 1C.

Fig. 3. The FTIR spectra of the samples prepared at different temperatures ranging from 250–450 1C (a–e). The curves have been vertically shifted to indicate them in a better manner.

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disappear. A similar process has been suggested by Gadalla [19]. During the thermal decomposition of nickel acetate, there is a possibility of the formation of acetic acid, ketone, formic acid, methane, ethanol and CO2 and H2 gas [20].

3.2. Structural properties The XRD spectra of the NiO films prepared at 250–450 1C is shown in Fig. 2. The films prepared at 250 1C exhibits an amorphous nature, while the films prepared at 300–450 1C shows the crystalline nature. The XRD spectrum reveals that the NiO films have cubic structure with peaks at (111), (2 0 0) and (2 2 0) planes. The growth along the (2 0 0) plane seems to be the preferred orientation of the films. The crystallite size was calculated using Debye–Scherrer formula in the (2 0 0) plane and is in the range 7–26 nm. The crystallinity of the films increases due to removal of interlayer water resulting from the decomposition of Ni(OH)2 into NiO. It may also be due to better homogeneity of the films. The first process seems to be appropriate as in evidence by FTIR studies discussed in the following section. Lin et al. [21] reported the similar structure for electrodeposited NiO films. Noh et al. [22] also reported the (2 0 0)

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plane to be the preferred orientation for the sputter deposited NiO films. The FTIR spectrum of the xerogel (KBr pellet method) treated at various temperatures is shown in Fig. 3. The film prepared at 250 1C exhibits absorption at 1032 cm1 and 1465 cm1 arising out of an acetate groups [23]. OH bending vibrations are observed at 1630 cm1 [24] and the band at 2380 cm1 is due to CO2 vibrations. The inset in Fig. 3 shows weak absorption at 668 cm1 for the xerogel treated at 250 1C which corresponds to d(Ni-OH) vibration. Heat The equation NiðOHÞ2 ! NiO þ H2 O explains the absence of OH peaks at higher temperatures (X300 1C) [9]. The intensity of the 1630 cm1 band decreases with temperature. In the case of xerogel treated at higher temperatures (400 and 450 1C), the peaks associated with OH and acetate groups disappear. The decomposition products probably leave the films making it free from OH and other groups with pure NiO film being left on the substrate. The bands below 500 cm1 [inset in Fig. 3 for the xerogel treated at 400 1C], correspond to the vibrations of Ni–O. The SEM images for the films prepared at different temperatures are shown in Fig. 4. The excellent morphological change has been observed due to increase in preparation temperature. Smooth morphology has been observed for the film prepared at

Fig. 4. The SEM images of the NiO films prepared at different temperatures.

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250 1C, while some aggregations were observed in the films prepared at 300 1C. The film prepared at a range 350–450 1C exhibit the formation of nanorods.

3.3. Electrochromic properties The electrochromic properties of the films are dependent on the structure and morphology of the films and in particular by the particle size. The films have been cycled from 0.25 V to 0.7 V at a sweep rate of 25 mV/s in 0.1 M KOH solution. The cyclic voltammetric curves [10th cycle] for the NiO films are shown in Fig. 5. The peaks associated with each cycle correspond to the oxidation and the reduction process during the electrochemical experiment. The diffusion coefficient has been estimated using the formula ip ¼ 2:72  105  n2=3  D1=2  C 0  v1=2

(a)

Here n is the number of electrons assumed to be 1 and C0 is the concentration of the active ions in the electrolyte, v the scan rate, ip the anodic/cathodic peak current and D the diffusion coefficient. The D-value is calculated using the relation (a). The anodic/ cathodic potential, peak current density and diffusion coefficient values are given in Table 1. The results clearly indicate that the films prepared at 300 1C with a thickness of 306 nm (calculated from optical transmission spectra, not shown) exhibit maximum anodic/cathodic diffusion coefficient value of 11 1012 cm2/s/ 6.44  1012 cm2. Wu et al. [25] reported the diffusion coefficient value of 1013 cm2/s for NiO films prepared via e-beam evaporation method. Our values are about an order of magnitude larger. The difference may be due to the smaller particle size (in the present work).

Different redox reactions have been proposed by various authors. Some of them are as follows: NiðOHÞ2 2NiOOH þ Hþ þ e

(1)

NiO þ OH 2NiOOH þ e

(2)

NiðOHÞ2 þ OH 2NiOOH þ H2 O þ e

(3)

NiII Ox Hy 2½NiII ð1zÞ NiIII z Ox HðyzÞ þ zHþ þ ze

(4)

NiO þ H2 O2NiOOH þ Hþ þ e

(5)

NiðOHÞ2 þ 3NiO2NiO þ Ni3 O4 þ 2Hþ þ e

(6)

Colourless

brown/black colour

where y is unknown. The extraction of proton and electron (Eq. (1)) [26–28], and hydroxyl insertion into nickel oxide lattice (Eq. (2)) model was proposed by Serebrennikova et al. [29] and Nagai [30], while Ottaviano et al [31] reported that Eq. (3) may be the reason for the EC effect. A different reaction (Eq. (4)) was proposed by Monk that includes both Ni2+ and Ni3+ states in the coloured state [32]. Azen et al. [33] proposed a reaction (Eq. (5)) involving water which is present in the film and is responsible for the electrochromic behaviour and is independent of Ni(OH)2 and OH. But Scarminio [34] tried to show that the water in the aqueous electrolyte has no role on the electrochromic behaviour (Eq. (6)). The electrochromic redox reactions for pulsed laser deposited NiO films were proposed by Bouessay et al. [16]. According to them in the beginning, Ni(OH)2 is formed chemically on the surface of NiO by immersion in KOH according to NiO+H2O2Ni(OH)2, then the reaction proceeds according to (2). In the present work, the CV spectrum exhibits three oxidation and reduction peaks. This suggests that, nickel in this film undergoes more than one oxidation/reduction process. Such a model (7) was proposed by Sampe et al. [35] for nickel oxide electrodes NiðOHÞ2 2NiOx ðOHÞ2x þ xHþ þ xe

(7)

where x can be as high as 1.6. This equation shows the oxidation of Ni2+ into higher oxidation states, Ni3+ and/or Ni4+. The oxidation product (NiOx(OH)2x) has been viewed as ½Ni2þ a Ni3þ b Ni4þ g O2 x ðOHÞ 2x ;

a þ b þ g ¼ 1 and 2a þ 3b þ 4g ¼ 2 þ x

Fig. 5. Cyclic voltammetric curves for the NiO films prepared at (a) 250 1C, (b) 300 1C, (c) 350 1C, (d) 400 1C and (e) 450 1C using 0.1 M KOH as electrolyte, Pt as the counter electrode and Ag/AgCl as a reference electrode.

Toumi [36] reported that the oxidized form contains Ni(IV). The Chronoamperometric curves for NiO films prepared at different temperatures (250–450 1C) are shown in Fig. 6. The spectrum reveals that the films prepared at higher temperatures yield a faster response than the films prepared at lower temperatures. A home made electrochromic response cell with 1 M NaOH as an electrolyte and a graphite bar as the counter

Table 1 The temperature depended VA, VC, ipA, ipC, DA, DC, DT values and response time for colouration/bleaching. Temp Anodic (1C) potential (V/ cm2) VA

Cathodic potential (V/ cm2) VC

Anodic current density (104 A) (ipA)

Cathodic current density (104 A) (ipC)

Anodic diffusion coefficient (1012 cm2/ s) (DA)

Cathodic diffusion coefficient (1012 cm2/ s).(DC)

Transmission change DT ¼ (TbTc)%

Time taken for colouration/ bleaching (s)

250 300 350 400 450

0.38 0.48 0.47 0.50 0.49

2.46 4.51 2.93 1.66 1.18

3.34 3.45 1.98 1.15 0.563

3.27 11.00 4.64 1.49 0.75

6.03 6.44 2.12 0.72 0.17

61 68 64 58 29

16/6.5 5.6/2.3 3.7/0.8 1.3/0.4 1.3/0.3

0.43 0.60 0.60 0.60 0.58

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1199

100 90 Transmisssion (%)

a

b

80 c

70

d

a

60

e

50 e

40 30

c b

20 10 0 15 Fig. 6. The Chronoamperometric curves for the NiO films.

20

25

30

35 40 45 Time (sec)

50

55

60

65

Fig. 9. The time response curve for the NiO films prepared at different temperatures (a) 250 1C, (b) 300 1C, (c)350 1C, (d) 400 1C and (e) 450 1C.

100

b1 b2

90 b3 Transmittance (%)

80

b4 b5

70 60

c5

50 c1

40

c4

30 20

c3

c2

10 0 300 350 400 450 500 550 600 650 700 750 800 850 900 950 Wavelength (nm) Fig. 7. The coloured (c) and bleached (b) states of the NiO films prepared at different temperatures (c1/b1) 250 1C, (c2/b2) 300 1C, (c3/b3) 350 1C, (c4/b4) 400 1C and (c5/b5) 450 1C.

0.7

5

0.6 PCR

4

0.5 0.4

3

0.3

2

0.2 1 0 250

0.1 300

350 Temperature (°C)

400

Change in optical density

0.8

6

0 450

Fig. 8. The photopic contrast ratio (PCR) and change in optical density as a function of film preparation temperature.

electrode was used to study the process of coloration/bleaching by applying 72 V. The optical transmission of the films in the coloured and bleached state is shown in Fig. 7. The film prepared at 300 1C exhibit the maximum variation in optical transmission between coloured and bleached state, DT550 nm ¼ 68%. The change in optical density is DOD550 nm ¼ log (Tbleached)log(Tcoloured) ¼ 0.71. The photopic contrast ratio (PCR) is (Tbleached/Tcoloured) ¼ 5.17. The DT, values for all the films are reported in Table 1 and the

changes in DOD and PCR values with respect to preparation temperature are shown in Fig. 8. To get information on the electrochromic response of the films with time, the optical transmission at 630 nm has been measured by applying square pulse of 71.8 V. The electrochromic response is shown in Fig. 9. The film prepared at 250 1C exhibits a slow response (about 50 s to reach saturation). The time responses for the films are given in Table 1. The response time decreases with increase of in preparation temperature. This is in good agreement with the CA results and the bleaching is faster than colouration. There is a decrease in the film colouration at higher temperatures due to the formation of compact crystalline nickel oxide. This can be attributed to the fact that ion insertion in nickel oxide takes place through defects based on absorption modulation. The films prepared at higher temperatures are too dense for ion intercalation. The crystalline NiO with a particle size of 7 nm prepared at 300 1C exhibits good EC performance, such small particles may increase the ion mobility during colouring and bleaching process. The nanorods, being rod like, probably act like a guide for ion transport, there by decreasing the ion mobility through the solid. For example, the SEM shows longer nanorods at higher temperatures. The change in transmission (DT) has gone from 64% for shorter rods at 350 1C to 29% for longer rods at 450 1C. The OH associated with Ni in the films seem to play an important role in the electrochromic behaviour, the OH was removed when the film is subjected to higher temperatures. The removal of OH and the increase of the particle size appear to have reduced the optical modulation capacity of the films at higher temperatures. To test the quality and degradation of the electrochromic behaviour of the NiO films with number of cycles of bleaching and colouration, transmission spectra were recorded continuously by the application of pulses 71.5 V with pulse duration of 15 s. These measurements were made on films prepared at 300 1C as they exhibited the best electrochromic behaviour. Fig. 10 shows the transmittance curve at 630 nm during the pulse potential cycling test for NiO film prepared at 300 1C for various cycles. The film exhibits the value of DT ¼ 38% for the initial cycles and the bleached state transmission goes on increasing and reached the maximum value of 85% at 191 cycles. It remains constant up to 384 cycles and slowly decreases there after, while at the same time the colouration (transmission) goes on decreasing with the cycling, this may be due to the increase in the cathodic coulometric capacity. There is a slight decrease in the bleached state transmittance than that observed at early cycles.

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90

90

80

80

70

70

Transmission (%)

Transmission (%)

1200

60 50 40 30 20

50 40 30 20 10

10 0

0 0

0.5

1

1.5

2 2.5 3 Time (min)

3.5

4

4.5

79 79.5 80 80.5 81 81.5 82 82.5 83 83.5 84 Time (min)

5

90

90

80

80

70

70

Transmission (%)

Transmission (%)

60

60 50 40 30 20

60 50 40 30 20

10 0 234 234.5 235 235.5 236 236.5 237 237.5 238 238.5 239 Time (min)

10 0

298 298.5 299 299.5 300 300.5 301 301.5 302 302.5 303 Time (min)

Fig. 10. The EC behaviour of the NiO film (prepared at 300 1C) depending upon the number of cycles [1 min corresponds to 4 cycles].

4. Conclusion

90 80 Tbleached

Transmission (T%)

70 60

ΔT

50 40 30 Tcoloured

20 10

Nanostructured NiO films have been coated on FTO glass substrate via sol–gel dip coating method. The preparation temperature plays a major role in the electrochromic behaviour by removing OH content at elevated temperatures and forming nanostructures. The NiO films prepared at 300 1C with the particle size of 7 nm exhibit better electrochromic behaviour compared with the amorphous and the nanorods of NiO films. This study indicates that the NiO films prepared at 300 1C are optimally suited for electrochromic applications with a DT of 68%, good reversibility and a colouration /bleaching response time of 5.6/ 2.3 s lasting beyond 1000 cycles.

0 0

100 200 300 400 500 600 700 800 900 1000 1100 1200 Number of cycles

Fig. 11. The bleached, coloured and change in transmission at 630 nm with respect to number of cycles for the films prepared at 300 1C.

This indicates that the reduction process is not fully completed and probably it is due to irreversible coulometric capacity [8]. The film shows a maximum variation in the transmittance of (DT) 63% at 367 cycles and retains the same up to 448 cycles. A rapid decrease in the bleached state transmission has been observed around at 952 cycle (DT ¼ 45%), after that it has retained the bleached state and shows DT ¼ 42% at 1200 cycle. The overall electrochromic performance and cyclic stability of the NiO film prepared at 300 1C is shown in Fig. 11.

Acknowledgement The authors wishes to thank Dr. S. John Abraham, Reader, Department of Chemistry, GRU for providing facilities for recording CV and Mr. A. Sivanesan and Mr. P. Kalimuthu, (Research Scholars Department of Chemistry, GRU, Gandhigram) for their help. References [1] G.A. Niklasson, C.G. Granqvist, Electrochromics for smart windows: thin films of tungsten oxide and nickel oxide, and device based on these, J. Mater. Chem. 17 (2007) 127–156. [2] P.C. Yu, C.M. Lampert, In-situ spectroscopic studies of electrochromic hydrated nickel oxide films, Sol. Energy Mater. 19 (1989) 1–16.

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